This invention relates to the masking of abrasion injury such as marks due to grazing, scratching, galling, scoring, and like abrasive action (hereinafter referred to collectively as "scuffs") on glass articles, particularly glass vessels, especially returnable glass vessels.
The glass vessels which are used for beer, refreshing beverages, milk and the like in some countries are generally returnable, and are recovered after use from the market and reused. The glass vessels such as bottles which are to be recovered and reused repeatedly come in contact with each other or, for example, with metallic articles, in the course of bottling processes or transportation, whereby scuffs are left on the surfaces of the glass vessels. As a result, the appearance of the glass vessels is markedly impaired, and the commodity value of the resulting bottled beverages is lowered.
Accordingly, it has been considered to coat a masking agent onto the scuffed parts on the surfaces of glass vessels to mask such scuffs. Some masking agents have been proposed but are not fully satisfactory as far as we are aware.
The reason for this is that a scuff-masking agent for the above mentioned purpose must simultaneously satisfy several requirements, but the known masking agents are not satisfactory in this respect.
In general, a coat film provided on the scuffed parts of a glass vessel must satisfy the following requirements. (1) The film must have good scuff-masking property. (2) It must have good water resistance. (3) It must have no surface tackiness. (4) The required properties such as hardness and strength of the film must be exhibited at temperatures in the vicinity of room temperature. (5) The masking material itself must not be toxic, and a solvent, if used, must not be toxic. (6) The films must be readily and completely removable by washing with an alkaline solution in a bottle-washing step or the like.
First of all, this film must, of course, have good scuff-masking property, but it also must have good water resistance. The glass vessels filled with beer, refreshing beverages and the like are often immersed in chilled water contained in a show-case or tank in summer. Thus, the coat film provided on a glass vessel is required to have high water resistance such that the film is not removed even when the vessel is immersed in water for several weeks. Moreover, such glass vessels are generally handled with bare hands. When surfaces of the film of the glass vessels are tacky, the handlers experience an unpleasant sensation, and the film surface may be contaminated with dust from the atmosphere. On the other hand, in the case where heating is necessary to have the coated films cured or crosslinked to obtain the properties required for the masking films such as hardness and strength, there is the risk of deterioration of the contents contained in the vessels since the coating with scuff-masking agent is normally carried out after the vessels are filled with the contents, because scuffs are sometimes produced in the filling step. This situation is the same as that in the case where irradiation with light rays is employed instead of heating.
Since such glass vessels accommodate foodstuffs, the coat film materials must be nonpoisonous and odorless, and a solvent to be used when a coat film is formed from a solution must also be nonpoisonous (this being also required from the point of view of the working environment). Thus, organic solvents other than alcohols should not be used. In other words, the film materials themselves or the precursors thereof should be soluble in alcohols, especially in ethanol.
Another important property required of the film is that it must have good removability in a bottle-washing step with an alkaline solution. In general, such glass vessels are collected and are then subjected to washing and sterilization in a bottle-washing machine with an alkaline solution, wherein, ordinarily, aqueous sodium hydroxide of about 2 to about 4% concentration is used and the operation is carried out for about 10 to about 20 minutes at a temperature of 60.degree. to 80.degree. C., the glass vessels thus washed being then reused. If the coat films are not completely stripped by the alkaline solution of the bottle-washing machine, and some parts thereof remain on the glass surfaces, the scuff-masking agent will be again put on the resulting uneven surfaces, whereby the aesthetic appearance of the coated surfaces will be impaired.
In this connection, a method of masking scuffs which comprises establishing strong films (permanent films) which cannot be removed with an alkaline solution on the scuffed surfaces of glass vessels has been proposed. However, since the glass vessels are repeatedly collected and reused, it is difficult to prevent scuffs from being produced on the permanent films themselves. Furthermore, when the vessels are repeatedly washed with an alkaline solution, the appearance of the films is apt to deteriorate. That is, the films may become white-opaque, for example. Moreover, a thicker film (50 .mu.m or more) is generally required to obtain such a permanent film, which entails a cost problem. To the best of our knowledge at present, the method of using permanent films is not practicable.
As mentioned above, there have been proposed several scuff-masking agents. In addition, variety of coating materials including scuff-preventing agents and breakage-preventing agents are known. All of these agents do not completely satisfy the above enumerated requirements. For example, (1) Surfactants have poor water resistance. When surfactants having an hydrophile-lipophile balance (HLB) of 9 or more are used, the resulting coat films are removed when immersed in water for about 4 to 5 hours. Generally, surfactants result in greater tackiness. (2) Liquid paraffins have very poor water resistance. In the case of solid paraffins (having melting points of 42.degree. C. or higher), the resulting coat films have good removability in a bottle-washing operation. The stripped material, however, floats as scum on the surface of the alkaline solution in the bottle-washing machine and is redeposited on the glass surfaces to interfere with the bottle-washing operation. (3) Organic high-molecular materials (resins) which are coated in the form of aqueous emulsions result in coat films which have very poor water resistance unless they are protected by heating or irradiation with light rays. When they are provided in the form of solutions dissolved in organic solvents, there exist problems such as poor removability of the resulting dried film with an alkaline solution in a bottle-washing machine and toxicity of the organic solvent used, although the resulting films have good water resistance and absence of surface tackiness. When the high-molecular materials are of the type which requires curing after coating, a considerably high heating temperature is required.